Various industries that manufacture and assemble machinery having close tolerance, reciprocating parts have recognized the importance of having precise alignment among the reciprocating parts and the stationary components. The importance of such alignment may be explained using the widely applicable example of a piston reciprocating in a cylindrical bore. The simplest example is a piston that is free to reciprocate within the cylindrical bore with its path of reciprocation in the bore not restricted by anything except the bore. The movement of such a piston is guided by the surrounding walls of the cylindrical bore so there is no need for alignment.
However, if such a piston also has an axial hole through the center and slides on an axially aligned rod passing through the hole, the path of reciprocation is determined by both the walls of the cylindrical bore and the rod. Therefore, if the rod upon which the piston slides is sufficiently out of alignment with the central axis of the cylindrical bore, then at some point during the stroke of the piston where the rod is displaced radially from the central axis, the clearance, which should be a constant throughout the length of the stroke, will become nonexistent and excessive contact will occur. Such contact is often very damaging to the equipment and extremely difficult to repair, if not irreparable. Additionally, such contact will often wear down the various components thereby reducing the efficiency of the machine or causing it to malfunction.
In the past, centering of such components was performed manually. Human errors, compounded with errors from manual measurement equipment, did not provide the level of precision that is required by today's tolerances. Additionally, such manual determinations are difficult and time consuming for even the most skilled machinist. Further, often the alignment needs to be performed to a greater accuracy than can be achieved by a purely mechanical device.
More recently, semi-automated centering devices have been developed but they generally include a multitude of gages, meters, and indicators that must be carefully attached to the bore and are often extremely sensitive. Further, it is often necessary for such gages, meters, and indicators to extend into the bore to perform their functions, making them difficult to use, adjust, and read. Advanced optical alignment systems do exist, however they often require precision optics to produce predetermined patterns of light and are often extremely sensitive and cost prohibitive.
Precise centering of reciprocating components within a cylindrical bore is a need often encountered in the field of free piston machines, particularly Stirling devices such as Stirling engines and coolers. The efficiency of such devices is dependent upon exacting tolerances between the stationary and reciprocating components. In fact, the radial clearance between such components is often 12 to 13 microns.
Free piston coolers operating with such close tolerances generally incorporate linear gas bearings as well as specially designed surface coatings, such as fluoropolymers, on the reciprocating components. The radial loading on the linear gas bearings is minimized by attempting to maintain a consistent clearance throughout the stroke of the reciprocating components. In order to maintain a consistent clearance throughout the stroke of a free piston machine, the rod that passes through a hole in the piston along which the piston slides, such as the displacer rod, must be perfectly centered within the bore. When the displacer rod is not perfectly centered, the piston sidewalls approach the bore walls at some point during the stroke thereby getting closer than the desired clearance and increasing the radial bearing load. Only a slight misalignment of the displacer rod may result in contact between the piston and the bore. Therefore, misalignment of the displacer rod results in reduced bearing life as well as undue wear on the various components of a free piston machine.
Accordingly, there is a need for a precise centering method and system that is economical, portable, and easy for an unskilled operator to use. While some of the prior art devices attempted to improve the state of the art of centering systems, none have achieved the beneficial attributes of the present invention. With these capabilities taken into consideration, the instant invention addresses many of the shortcomings of the prior art and offers significant benefits heretofore unavailable.